从后台任务调用-setneedsDisplay是否有问题?

我有一个更新视图的后台任务。该任务调用-setneedsDisplay以绘制视图。

这工作:

- (void) drawChangesTask;
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    if (pixels) {
        drawChanges((UInt32 *) origPixels, (UInt32 *) pixels, CGBitmapContextGetBytesPerRow(ctx)/4, CGBitmapContextGetHeight(ctx), count--);

        if (count < 0) {
            count = 150;
        }
        else
            [self performSelectorInBackground:@selector(drawChangesTask) withObject:nil ];

        [self performSelectorOnMainThread:@selector(setNeedsDisplay) withObject:nil waitUntilDone:NO ];

    }
    [pool release];
}

这不起作用:

- (void) drawChangesTask;
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    if (pixels) {
        drawChanges((UInt32 *) origPixels, (UInt32 *) pixels, CGBitmapContextGetBytesPerRow(ctx)/4, CGBitmapContextGetHeight(ctx), count--);

        if (count < 0) {
            count = 150;
        }
        else
            [self performSelectorInBackground:@selector(drawChangesTask) withObject:nil ];

        [self setNeedsDisplay];

    }
    [pool release];
}

有人知道为什么吗?当我说它不起作用的时候,我的意思是它运行了几十次迭代,有时我看到我的图像部分上下移动,或者完全空白,然后Deugger给了我一个 EXC_BAD_ACCESS 在核心图形的某个地方。

另外,如果我自己不处理自动释放池,那么我会收到泄漏的错误消息。也不明白为什么会这样。我的drawChanges()不会创建任何新对象。错误如下:

2009-08-17 11:41:42.358 BlurApp[23974:1b30f] *** _NSAutoreleaseNoPool(): Object 0xd78270 of class NSThread autoreleased with no pool in place - just leaking